This section provides background information related to the present disclosure which is not necessarily prior art.
Head up displays (HUD) are known and are currently used in many military and commercial aircraft. HUDs are also finding application in automobiles. Conventionally, a HUD unit is mounted in the automobile to project an image in front of the driver. Adapting HUD units for use in automobiles necessitates making the units rugged, reliable, cost effective, and functional in a wide range of environmental and ambient lighting conditions.
One problem with HUDs in automobiles is maintaining satisfactory image illumination over a wide range of ambient lighting conditions. These ambient lighting conditions span from the extremely bright daylight lighting conditions of facing direct sunlight to the very dim nighttime lighting conditions of unlit rural highways. Daylight conditions generally require a bright or high intensity light source so that the projected image is visible in the corresponding bright ambient light conditions. Conversely, nighttime or low light conditions require a dim or low intensity light source to provide a projected image that is safe and comfortable in the corresponding dim ambient light conditions.
In order to meet the demands of bright or high intensity daylight conditions encountered by automobiles, the light source of a HUD can achieve very high intensities. For example, some automobile head-up displays have an image light source that can achieve an intensity of 8,000 candela per square meter or more. This light intensity is great enough to distract a driver, or even temporarily cause night blindness, depending upon the surrounding ambient lighting conditions. Not only can such over-intensity situations be detrimental to the reliability and service life of the HUD, but they can also cause unsafe driving situations.